The frequency of radio frequency voltage controlled oscillators (RF VCO) has been closely controlled by phase locking a feedback signal from the RF VCO to a crystal controlled reference oscillator (XO). A phase detector has been used to determine the phase difference between the feedback signal and a crystal controlled reference frequency; and an integrator has been used to summate the phase difference and to control the frequency of the RF VCO oscillator in accordance with the summated phase difference.
Improvements taught by the prior art over the basic phase locked oscillator include the use of prescalers to provide a feedback signal having a lower frequency than the RF VCO, thereby lowering the required frequency of the controlling circuitry. Prior art improvements over the basic circuitry also include the use of a dual modulus divider to channelize the output frequency by dividing the feedback by higher and lower dividing ratios in a technique known as pulse swallowing. That is, a pulsed signal, having a frequency proportional to the output frequency, is provided in the feedback path, and channelizing is accomplished by swallowing, or removing, pulses in the feedback path.
While phase locked oscillators have provided a frequency output that drifts very little, a significant problem has been in trying to frequency modulate the output on a D.C. basis.
One attempt at D.C. modulating the frequency output of a phase locked oscillator has been to use a voltage controlled crystal oscillator (VCXO) in place of a crystal controlled reference oscillator (XO), and to simultaneously modulate the RF VCO as well as the VCXO. The problems with this approach have been non-linearities in the VCXO, limited frequency deviation, limited frequency response of modulation, and significantly increased frequency drift as a function of both time and temperature.
In stark contrast to the limitations of the prior art, the present invention provides highly linear D.C. modulation of a RF VCO together with very little frequency drift as well as almost unlimited deviation and frequency response.
More particularly, the present invention provides both single frequency and channelized phase locked loops that are capable of D.C. modulation.